1. Field of the Invention
The present invention relates to a strip dual mode loop resonator utilized to resonate waves in frequency bands ranging from an ultra high frequency (UHF) band to a super high frequency (SHF) band, and relates to a band-pass filter composed of a series of resonators which is utilized as a communication equipment or measuring equipment.
2. Description of the Related Art
A half-wave length open end type of strip ring resonator has been generally utilized to resonate microwaves ranging from the UHF band to the SHF band. Also, a one-wave length strip ring resonator has been recently known. In the one-wave length strip ring resonator, no open end to reflect the microwaves is required because an electric length of the strip ring resonator is equivalent to one-wave length of the microwaves. Therefore, the microwaves are efficiently resonated because electric energy of the microwaves resonated is not lost in the open end.
In addition, in cases where a band-pass filter is composed of a plurality of strip ring resonators arranged in series, a strip dual mode ring resonator functioning as a two-stage filter is required to efficiently filter the microwave in the band-pass filter.
2-1 Previously Proposed Art
A first conventional resonator is described.
FIG. 1A is a plan view of a one-wave length strip ring resonator in which no open end is provided. FIG. 1B is a sectional view taken generally along the line I--I of FIG. 1A. Each of constitutional elements of the ring resonator shown in FIG. 1A is illustrated in FIG. 1B.
As shown in FIG. 1A, a one-wave length strip ring resonator 11 conventionally utilized is provided with an input strip line 12 in which microwaves are transmitted, a closed ring-shaped strip line 13 in which the microwaves transferred from the input strip line 12 are resonated, and an output strip line 14 to which the microwaves resonated in the strip ring 13 are transferred.
As shown in FIG. 1B, the input and output strip lines 12, 14 and the ring-shaped strip line 13 respectively consist of a strip conductive plate 15, a dielectric substrate 16 surrounding the strip conductive plate 15, and a pair of conductive substrates 17a, 17b sandwiching tile dielectric substrate 16.
The ring-shaped strip line 13 has an electric length equivalent to a wavelength of the microwave. The electric length of the ring-shaped strip line 13 is determined by correcting a physical line length of the ring-shaped strip line 13 with a relative dielectric constant .epsilon..sub.r of the dielectric substrate 16.
The input strip line 12 is arranged at one side of the strip ring 13 and is coupled to the ring-shaped strip line 13 in capacitive coupling. That is, when the microwaves transmit through the input strip line 12, electric field is induced in a gap space between the input strip line 12 and the ring-shaped strip line 13. Therefore, the intensity of electric field in the ring-shaped strip line 13 is also increased at a coupling point P1 adjacent to the input strip line 12 to a maximum value.
The output strip line 14 is arranged at an opposite side of the strip ring 13. In other words, the output strip line 14 is spaced 180 degrees (a half-wave length of the microwaves) in the electric length apart from the input strip line 12. In this case, the intensity of the electric field in the ring-shaped strip line 13 is maximized at a coupling point P2 adjacent to the output strip line 14 because the output strip line 14 is spaced 180 degrees in the electric length apart from the input strip line 12. Therefore, the output strip line 14 is electrically coupled to the ring-shaped strip line 13 in capacitive coupling.
In the above configuration, when microwaves are transmitted in the input strip line 12, electric field is induced at a gap portion between the input strip line 12 and the ring-shaped strip line 13 by the microwaves. Therefore, the intensity of the electric field in the ring-shaped strip line 13 is maximized at the coupling point P1 adjacent to the input strip line 12. Thereafter, the electric field induced at the coupling point P1 is diffused into the ring-shaped strip line 13 as traveling waves. In other words, the microwaves are transferred from the input strip line 12 to the ring-shaped strip line 13. In this case, a part of the travelling waves are transmitted in a clockwise direction, and a remaining part of the travelling waves are transmitted in a counterclockwise direction. In cases where the wavelength of the microwaves is equivalent to the electric length of the ring-shaped strip line 13, the microwaves are resonated in the ring-shaped strip line 13. Therefore, the intensity of the microwaves in the ring-shaped strip line 13 is amplified.
Thereafter, the intensity of the electric field in the ring-shaped strip line 13 is maximized at the coupling point P2 adjacent to the output strip line 14 because the output strip line 14 is spaced 180 degrees in the electric length apart from the input strip line 12. Therefore, the electric field is induced at a gap space between the ring-shaped strip line 13 and the output strip line 14. As a result, the microwave resonated in the ring-shaped strip line 13 is transferred to the output strip line 14.
Accordingly, the strip ring resonator 11 functions as a resonator of the microwaves.
In this case, the microwaves can be resonated in the strip ring 13 even though the electric length of the ring-shaped strip line 13 is an integral multiple of the wavelength of the microwaves.
The strip ring resonator 11 is often utilized to estimate the dielectric substrate 16 because a resonance frequency (or a central frequency) of the microwaves is shifted according to a physical shape of the dielectric substrate 16 and the relative dielectric constant .epsilon..sub.r of the dielectric substrate 16.
The strip ring resonator 11 is described in detail in the literature "Resonant Microstrip Ring Aid Dielectric Material Testing", Microwaves & RF, page 9-102, April, 1991.
2-2 Another Previously Proposed Art
A second conventional resonator is described.
FIG. 2 is a plan view of a strip dual mode ring resonator functioning as a two-stage filter.
As shown in FIG. 2, a strip dual mode ring resonator 21 conventionally utilized is provided with an input strip line in which microwaves are transmitted, a one-wave length strip ring 23 electrically coupled to the input strip line 22 in capacitive coupling, and an output strip line 24 electrically coupled to the strip ring 23 in capacitive coupling.
The input strip line 22 is coupled to the strip ring 23 through a gap capacitor 25, and the output strip line 24 is coupled to the strip ring 28 through a gap capacitor 26. Also, the output strip line 24 is spaced 90 degrees (or a quarter-wave length of the microwaves) in the electric length apart from the input strip line 22.
The strip ring 23 has an open end stub 27 in which the microwaves are reflected. The open end stub 27 is spaced 135 degrees (or 3/8-wave length of the microwaves) in the electric length apart from the input and output strip lines 22, 24.
In the above configuration, the action of the strip dual mode ring resonator 21 is qualitatively described in a concept of travelling waves.
When travelling waves are transmitted in the input strip line 22, electric field is induced in the gap capacitor 25. Therefore, the input strip line 22 is coupled to the strip ring 23 in the capacitive coupling, so that a strong intensity of electric field is induced at a point P3 of the strip ring 23 adjacent to the input strip line 22. That is, the travelling waves are transferred to the coupling point P3 of the strip ring 23. Thereafter, the travelling waves are circulated in the strip ring 23 to diffuse the electric field strongly induced in the strip ring 23. In this case, a part of the travelling waves are transmitted in a clockwise direction and a remaining part of the travelling waves are transmitted in a counterclockwise direction.
An action of the travelling waves transmitted in the counterclockwise direction is initially described.
When the travelling waves transmitted in the counterclockwise direction reach a coupling point P4 of the strip ring 23 adjacent to the output line 24, the phase of the travelling wave shifts by 90 degrees. Therefore, the intensity of the electric field at the coupling point P4 is minimized. Accordingly, the output strip line 24 is not coupled to the strip ring 23 so that the travelling waves are not transferred to the output strip line 24.
Thereafter, when the travelling waves reach the open end stub 27, the phase of the travelling wave further shifts by 135 degrees as compared with the phase of the travelling wave reaching the coupling point P4. Because the open end stub 27 is equivalent to a discontinuous portion of the strip ring 23, a part of the travelling waves are reflected at the open end stub 27 to produce reflected waves, and a remaining part of the travelling waves are not reflected at the open end stub 27 to produce non-reflected waves.
The non-reflected waves are transmitted to the coupling point P3. In this case, because the phase of the non-reflected waves transmitted to the coupling point P3 totally shifts by 360 degrees as compared with that of the travelling waves transferred from the input strip line 22 to the coupling point P3, the intensity of the electric field at the coupling point P3 is maximized. Therefore, the input strip line 22 is coupled to the strip ring 23 so that a part of the non-reflected waves are returned to the input strip line 22. A remaining part of the non-reflected waves are again circulated in the counterclockwise direction so that the microwaves transferred to the strip ring 23 are resonated.
In contrast, the reflected waves are returned to the coupling point P4. In this case, the phase of the reflected waves at the point P4 further shifts by 135 degrees as compared with that of the reflected wave at the open end stub 27. This is, the phase of the reflected wave at the point P4 totally shifts by 360 degrees as compared with that of the travelling waves transferred from the input strip line 22 to the coupling point P3. Therefore, the intensity of the electric field at the coupling point P4 is maximized, so that the output strip line 24 is coupled to the strip ring 23. As a result, a part of the reflected wave is transferred to the output strip line 24. A remaining part of the reflected wave is again circulated in the clockwise direction so that the microwave transferred to the strip ring 23 is resonated.
Next, the travelling waves transmitted in the clockwise direction is described.
A part of the travelling waves transmitted in the clockwise direction are reflected at the open end stub 27 to produce reflected waves when the phase of the travelling waves shifts by 135 degrees. Non-reflected waves formed of a remaining part of the travelling waves reach the coupling point P4. The phase of the non-reflected waves totally shifts by 270 degrees so that the intensity of the electric field induced by the non-reflected waves is minimized. Therefore, the non-reflected waves are not transferred to the output strip line 24. That is, a part of the non-reflected waves are transferred from the coupling point P3 to the input strip line 22 in the same manner, and a remaining part of the non-reflected waves are again circulated in the clockwise direction so that the microwave transferred to the strip ring 23 is resonated.
In contrast, the reflected waves are returned to the coupling point P3. In this case, because the phase of the reflected waves at the coupling point P3 totally shifts by 270 degrees, the intensity of the electric field induced by the reflected waves are minimized so that the reflected waves are not transferred to the input strip line 22. Thereafter, the reflected waves reach the coupling point P4. In this case, because the phase of the reflected waves at the coupling point P4 totally shifts by 360 degrees, the intensity of the electric field induced by the reflected waves is maximized. Therefore, a part of the reflected waves are transferred to the output strip line 24, and a remaining pare of the reflected waves are again circulated in the counterclockwise direction so that the microwaves transferred to the strip ring 23 are resonated.
Accordingly, because the microwaves can be resonated in the strip ring 23 on condition that a wavelength of the microwaves equals the electric length of the strip ring 23, the strip dual mode ring resonator 21 functions as a resonator and a filter.
Also, the microwaves transferred from the input strip line 22 are initially transmitted in the strip ring resonator 23 as the non-reflected waves, and the microwaves are again transmitted in the strip ring resonator 23 as the reflected waves shifting by 90 degrees as compared with the non-reflected waves. In other words, two orthogonal modes formed of the non-reflected waves and the reflected waves independently coexist in the strip ring resonator 23. Therefore, the strip dual mode filter 21 functions as a dual mode filter. That is, the function of the strip dual mode filter 21 is equivalent to a pair of a single mode filters arranged in series.
In addition, a ratio in the intensity of the reflected waves to the non-reflected waves is changed in proportional to the length of the open end stub 27 projected in a radial direction of the strip ring resonator 23. Therefore, the intensity of the reflected microwave transferred to the output strip line 24 can be adjusted by trimming the open end stub 27.
The strip dual mode ring resonator 21 is proposed by J. A. Curtis "International Microwave Symposium Digest", IEEE, page 443-446 (N-1), 1991.
2-3 Problems to be Solved by the Invention
However, there are many drawbacks in the strip ring resonator 11. That is, it is difficult to manufacture a small-sized strip ring resonator 11 because a central portion surrounded by the ring-shaped strip line 13 is a dead space. Also, the electric length of the ring-shaped strip line cannot be minutely adjusted after the ring-shaped strip line 13 13 is manufactured according to a photo-etching process or the like. In this case, the resonance frequency of the microwaves depends on the electric length of the ring-shaped strip line 13. Therefore, the resonance frequency of the microwaves cannot be minutely adjusted. In addition, in cases where a plurality of strip ring resonators 11 are arranged in series to compose a band-pass filter, it is difficult to couple the ring-shaped strip lines 13 to each other because the ring-shaped strip lines 13 are curved.
Also, there are many drawbacks in the strip ring resonator 21. That is, a central frequency of the microwaves filtered in the strip ring resonator 21 cannot be minutely adjusted because the central frequency of the microwaves depends on the width of the open end stub 27 extending in a circumferential direction of the strip ring 23. Therefore, the central frequency of the microwaves manufactured does not often agree with a designed central frequency. As a result, a yield rate of the strip ring resonator 21 is lowered.
Also, because a resonance width (or a full width at half maximum) can be adjusted only by trimming the length of the open end stub 27, the resonance width cannot be enlarged. In other words, in cases where the width of tile open end stub 27 in the circumferential direction is widened to enlarge the resonance width, the phase of the reflected waves reaching the output strip line 24 undesirably shifts. As a result, the intensity of the microwaves transferred to the output strip line 24 is lowered at the central frequency of the microwaves resonated. Accordingly, in cases where a plurality of strip ring resonators 21 are arranged in series to compose a band-pass filter, the filter is limited to a narrow passband type of filter.